This invention relates to an electric heating fabric having opposed selvedges, said fabric being provided with electrically conductive lead wires of which at least one is arranged in each of said selvedges.
For heating large areas, such as frozen ground or runways, the method generally used hitherto is to heat air or water locally using conventional heaters. The air or water is then supposed to transport heat to the entire area and to transfer heat to its surface. This method may be called "indirectly heating a large area by means of convection".
It is however advantageous to provide a method for "directly heating" areas by means of "generated heating power", making use of the phenomenon that electric power is converted into heating power inside electrically conductive material if it is connected to a current source, and that this heating power becomes available at its outside surface area.
If such a heating element is available, the user has to know its electrical resistance, in order to establish its heating power in connection with the voltage to be used, and has to know also the size of its surface area in order to calculate the "density of power" for heating for establishing its temperature rise. He therefore has to know also the numerical relationship between power density, temperature rise and the thermal resistance value of its environment given by the so-called "thermal law of Ohm".
It is generally a problem to manufacture a heating element having the required size and resistance as well as the desired voltage rating. This problem is solved by the invention not only for heating large areas but also for manufacturing smaller heating elements for apparatus needing such heating elements.
The heating element of the invention can be mass produced, enabling the user to determine its size and obtaining the resistance as required, and subsequently to establish the heating power required, and its temperature rise as well enabling him to control its heating temperature accurately.
Electric heating elements having electrically conductive heating wires therein are known from the U.S. Pat. No. 3,060,303. Said known heating elements may be made with a predetermined heating capacity by molding under pressure and heat a mass of electrically conductive plastic or elastomer material into a suitable fabric tape, web or mat having electrically conductive leads therein to achieve a substantial integration with the tape, web or mat whereby a conduction approaching homogenity is achieved between the mass of material and the tape, web or mat.
A disadvantage of said known heating elements is that the fabrication thereof is cumbersome, takes much time and is expensive because special apparatus is necessary for molding under pressure and heat the mass of electrically conductive plastic or elastomer material and for curing said molded structure.
From such an electric heating fabric tape or web heating pieces forming electric heating elements may be cut so as all to have the same dimensions and consequently the same heating capacity.
However a very serious disadvantage of such an electric heating tape of fabric is that when heating elements having different heating capacities must be manufactured of the same tape or web this can only be achieved by increasing or decreasing their dimensions (length). When increasing their dimensions in order to have more heating capacity the disadvantage arises that they are too cumbersome so that they cannot be mounted in the apparatus for which they are destined. When decreasing their dimensions, if less heating capacity is necessary, the disadvantage may arise that they are too small to equally heat the surface of the object to be heated.
According to the method of manufacturing heating elements, by cutting pieces from an electric heating fabric in the shape of a tape or web, which have properties which are different one from another it is necessary to vary the size of mesh and filaments comprising the fabric tape or web, as well employing conductive elastomers whose electrical properties are different from one another so that for each type of element to have a predetermined surface area and a predetermined heating capacity a special fabric heating tape or web must be manufactured. In consequence thereof a large number of different electric heating tapes or wires must be kept in stock.
It is a general object of the present invention to provide an electric heating fabric which has the surprising advantage that pieces having one and the same surface area may be cut therefrom to form electric heating elements the heating capacity of which may be controllably varied in a very simple way within broad limits which are defined by the resistance of said elements, in the case that the strips of said elements containing electric heating wires are all connected in parallel and all connected in series respectively. As a consequence thereof, electric heating elements having a predetermined surface and a predetermined heating capacity as well as heating elements having a predetermined surface and a different predetermined heating capacity may be cut from one and the same electric heating fabric.
Electric power will be converted 100% into heating power in conductive material connected to an electric current source. The numerical relationship between power voltage and resistance is:
Q=(U.sup.2 /R) watt, PA1 U=voltage, PA1 R=resistance in Ohm, PA1 Q=heating power (watt) PA1 q=(Q/F) W/sq. mtr., q being the density of power.
If the outside surface area of the element is known to be F sq. mtr., the numerical relationship between power density (watt density), heating power and F will be:
Then the thermal law of Ohm represents the numerical relationship between density of power (q), temperature rise (.DELTA.t) and the thermal resistance value of the environment (R.sub.th), being: ##EQU1##
Then the heating temperature t of the entire area can be controlled, by one sensor and a power control device.
It is of importance to know that this law does not give the required information to be made comparable to the area to be heated, for heat to be transferred by convection. In this case the "driving force" for transferring heat is (t.sub.1 -t.sub.2), the difference of the temperature of the heater and the air (water), and is not accurately known, while heating. "Generated power" on the other hand gives rise to the temperature rise (.DELTA.t) and its density of power can be accurately calculated, measured and controlled.